"Complex behaviors, like the behaviors we call 'personality' or 'temperament,' can be associated -- genetically correlated -- with other traits that one might think are independent of such behaviors, like body shape and swimming abilities," Brian Langerhans, an assistant professor of biological sciences at North Carolina State University, said in a news release. "In other words, traits that seem unrelated may not be unrelated."

Researchers often measure an animal's relative "boldness" or "shyness" by how long they take to acclimate or respond to a new object or new environs. In this instance, researchers measured each fish's boldness by the amount of time the fish remained motionless after being placed in new surroundings.

The boldest fish were selectively bred to yield bolder and bolder offspring. In doing so, the researchers found they had also bred for two other physiological traits. The bolder offspring were becoming more sleek and slender in shape and also showed improved agility, darting more quickly through the water.

"We think pleiotropy, or one gene affecting two or more phenotypes, may explain the correlation between personality and locomotion," Langerhans explained. "On the other hand, the association between personality and body shape seems to reflect linkage disequilibrium that is not caused by pleiotropy or physical genetic linkage."

In other words, the two traits -- being bolder and more agile -- may help the fish make adulthood and reproduce, thus linking the traits via natural selection.

"This is one of the first studies linking personality variation to these other types of traits, and I think many more will emerge in the coming years," Langerhans said.